Stanley, the Iranians do not intend to start producing warhead quantities of highly-enriched uranium with the two or three thousand centrifuges they have apparently set up at Natanz. They would have to spin them continuously for a year or two just to get enough for a single warhead, and that golden egg would do them no good without testing it. What worries us is that once they figure out how to make lightly-enriched uranium with a few thousand centrifuges, they will know almost everything they need to know to make highly-enriched uranium in a cascade of 30,000 or 60,000 centrifuges — whether at Natanz or at a secret facility somewhere else. In other words, what is going on at Natanz now is a final dress-rehearsal for large-scale production of weapons-grade fissionable material—the key component for the serial production of nuclear warheads.

Estimates of the time it will take for Iran to make a warhead should be almost totally irrelevant to our consideration of what to do about Iran’s nuclear program — because that time horizon has no bearing on when the window for effective self-defense will open and close. If they pull out of the nonproliferation regime, and we lose what transparency we have into their nuclear activities, we will be living with the fear of nuclear terrorism for a long time to come, while proliferation in the Middle East and elsewhere spins out of control. Whether this state of affairs begins in five years or two years or ten is to my mind immaterial , if we make it inevitable. If our purpose were to ruin the 21st century for our children and grandchildren, we could hardly do better than to let Iran pull a veil of secrecy over its activities at the precise moment it’s ready to start enriching bomb-scale quantities of highly enriched uranium.

In June 1994, when the North Korea nuclear crisis nearly triggered a war with the United States, North Korea was still years away from being able to manufacture a warhead even in theory. So why did President Clinton nearly destroy the Yongbyon reactor in a textbook application of “Bush” preemption doctrine? Because North Korea announced its withdrawal from the International Atomic Energy Agency, which meant that the IAEA would no longer be able to provide assurances that the program was peaceful. It was the last point in time at which we would know the scope of North Korea’s nuclear activities with any certainty. After the loss of transparency, we would have to accept that, over time, whether quickly or slowly, our cities would become increasingly vulnerable and our national security increasingly precarious.

In a nuclear crisis such as that of Iran or North Korea, the window for effective counter-measures opens and closes long before they are able to make a warhead. What’s to be prevented is the loss of transparency at an early stage.